Ok, well I may not need an entire different default.aspx page.
But I've previously used Wordpress with clients and used some theme but the theme hasn't been exactly how I or the client want it to be.
So I always have to customize the theme pages and classes heavily.

I just want the option to do some serious customization, my own layout and easily add own classes for layout.

That's exactly you can do from Orchard themes. Orchard gives you complete control over the markup and you have the ability to override every piece of the layout in a fine-grained manner. Take a look at the docs, its well explained there or
watch some videos.

Also although Orchard uses Razor templates by default you can also use aspx ones. However Razor is a much more streamlined syntax so I encourage you to use that (if you don't know it with a knowledge of C# it takes about 30 seconds to start with it :-)).

If you know Wordpress development, Orchard development is pretty much nothing like it. I don't mean this in any degrading way but you have to think with an open mind.